Cochlear Morphology and Its Influence on Electrode Array Proximity to Modiolus

As the number of cochlear implant (CI) surgeries increases, greater emphasis should be placed on incremental improvements and on planning for a patient-centered approach. The shape of the basal turn (BT) of the cochlea has recently been evaluated and classified as either elliptical or round. However, the electrode position inside different shapes was not investigated.

This study aims to evaluate the impact of different cochlear shapes on the electrode-to-modiolus distance (EMD) among the CI patients.

A retrospective study was conducted on patients with normal cochleae who underwent cochlear implantation. Postoperative imaging was reconstructed using dedicated Otological planning software to assess cochlear morphology and electrode positioning. The cochlear shape was quantified using the basal turn width-to-height ratio (B/A ratio), and the EMD distance was measured.

The final analysis included 41 ears. The mean EMD was 3.3 mm in the basal electrodes, decreasing to 2.8 mm in the middle region and reaching 1.9 mm at the apical electrodes. The mean basal turn width-to-height ratio (B/A) was 0.8 (range: 0.7 to 0.9). Simple linear regression revealed that cochleae with a round-shaped basal turn (B/A >0.75) had a slightly shorter, but comparable, EMD in the basal region compared with those with an elliptical shape (3.3 vs. 3.5 mm). The apical region showed a bigger but nonsignificant distance in round-shaped cochleae (2.0 vs. 1.8 mm). In addition, impedance measurements at the 12-month interval showed a significant difference in basal electrode impedance between round and elliptical cochleae.

This study reported no statistically significant association between cochlear anatomic parameters and electrode-to-modiolus distance. Lateral wall electrodes demonstrated comparable distances to the modiolus in both elliptical and round-shaped basal turn cochleae, suggesting stable positioning across anatomic variations. Although descriptive trends were observed, these findings should be interpreted as exploratory. Larger prospective studies are needed to further evaluate the influence of cochlear morphology on electrode positioning across different electrode designs.